Fuel tanks on vehicles are subject to a number of safety requirements. These requirements include a venting system for relieving pressure in the tank when such pressure exceeds a predetermined level and a thermal relief system for preventing explosions when the temperature in the tank becomes excessive.
One known type of venting system is a pressure relief valve having a valve plug that is positioned outwardly of a vent opening in the valve housing and is spring biased to close the opening. A valve stem is attached to the plug and extends inwardly therefrom into the tank. The inner end of the stem carries a spring abutment for one end of a compression spring. The other end of the spring abuts the valve housing around the vent opening. The spring is calibrated to allow the valve plug and stem to move outwardly to relieve pressure when the pressure exceeds a predetermined level and to reseat the valve plug to close the vent opening when the pressure drops back down to an acceptable level. This valve arrangement has the advantages of simplicity of construction, reliability, and low cost.
One approach to providing thermally activated pressure relief is the use of a fusible material that melts at elevated temperatures to cause a vent opening or openings to be uncovered. The fusible material may itself be the cover for the opening, or it may be used to attach a nonfusible material cover to another portion of the venting structure. The latter arrangement has the advantages of allowing a relatively large opening to be uncovered very quickly and of requiring only small amounts of fusible material.
The separate provision of a pressure relief valve and thermal relief based on the melting of fusible material tends to complicate the overall structure of a safety system. Forming the stem from inner and outer portions which are held together by a fusible material permits the system's structure to be simplified. An example of this approach is disclosed in U.S. Pat. No. 4,732,188, granted Mar. 28, 1988, to Z. Gabrlik et al., and assigned to the assignee of the present application. In that patent, a major problem with the approach was recognized. This problem is that the stem in such a valve arrangement is subjected to significant tension forces by the biasing means, proportional to the amount of pressure required to activate the valve to vent the tank. Known fusible materials that are practical for use in a tank thermal relief system do not have sufficient strength to maintain their structural integrity at realistic venting pressures. Therefore, fusible materials are not practical for use as a direct load-carrying link in a valve stem. The solution to this problem arrived at in the above-cited patent was to arrange the inner and outer portions of the stem in a mechanically interlocking, load-transmitting position and to use the fusible material to maintain the stem portions in this position. In two embodiments disclosed in the patent, fusible material plugs extended radially through adjacent aligned openings in the inner and outer portions to hold the inner and outer portions together. In a third embodiment, the inner portion is formed of two legs, and the fusible material is positioned radially between the two legs.
U.S. Pat. No. 1,686,875, granted Oct. 9, 1928, to I. I. Nelson et al., discloses a spring biased pressure relief valve for a boiler. The valve has an inner chamber and an outer chamber. A valve element closes a passageway between the two chambers. Pressure in the boiler exceeding a predetermined amount causes the valve element to move upwardly to open the passageway to allow fluid to move from the inner chamber to the outer chamber and out through an outlet. A valve stem extends upwardly from the valve element. A spring positioned externally of the valve casing is positioned between the head of an adjustment screw and the upper end of the stem to bias the valve element into a closed position. The upper portion of the stem which abuts the spring is fusible to melt at a predetermined external temperature to release the spring. This is described as permitting the valve to operate independently of excess pressure in the boiler.
U.S. Pat. No. 3,040,764, granted June 26, 1962, to A. R. Feinberg, discloses a relief valve for fluid storage tanks, such as hot water tanks, which is actuated in response to predetermined fluid temperature or fluid pressure conditions within the tank. The valve includes a rod that extends through the valve seat and terminates at its inner end in a chisel-like point that is urged against a fusible body. A spring is positioned between an abutment carried by the rod and an abutment carried by the valve element to hold the valve closed. High temperature conditions cause the fusible body to melt to allow the rod to move inwardly and thereby release the spring and allow the valve to open.
U.S. Pat. No. 1,724,878, granted Aug. 13, 1929, to J. A. Jensen, discloses a vent device for tanks of vehicles that transport liquid fuels and the like in bulk. A top disk that forms a cover melts under fire conditions to allow a ball valve element to be expelled. A second ball valve element seats in an overturn condition. U.S. Pat. No. 4,458,711, granted July 10, 1984, to F. S. Flider, discloses a vent valve with a plastic housing which has a thin walled break-away top portion to provide an enlarged aperture under severe over-pressurization and which melts under severe fire conditions to provide an even larger aperture for maximum venting. C. J. Green discloses vehicle fuel tank valves for relieving pressure and preventing liquid spillage in U.S. Pat. No. 4,325,398, granted Apr. 20, 1982; No. 4,457,325, granted July 3, 1984; and No. 4,487,215, granted Dec. 11, 1984. Each of these three patents also discloses the use of fusible material for attaching a portion of the valve housing that covers a large vent opening to the rest of the housing. U.S. Pat. No. 2,489,787, granted Nov. 29, 1949, to E. W. Knowlton, discloses a safety cap for tanks or containers used for storing fluids like gasoline. The cap has a ball valve which relieves pressure. A fusible disk melts under high temperatures to uncover vent openings. The melting of the disk does not directly affect the functioning of the valve.
U.S. Pat. No. 3,918,606, granted Nov. 11, 1975, to R. D. Keller, discloses a fuel tank filler cap with a pressure relief valve having a spring biased valve plug and a stem which carries a spring abutment. The cap body has a separate vent valve with a ball that rolls in a retainer to close the vent opening when the cap is in a tilted orientation.
U.S. Pat. No. 2,357,681, granted Sept. 5, 1944, to C. R. Morton, discloses a fire detecting and alarm device in which each end of a spring is held down by a strip with a section of fusible material that melts at a predetermined temperature to release the spring. The manner in which the section of fusible material is incorporated is not disclosed. The device also has plugs that are held in openings in containers of fire extinguishant by fusible material that melts to release the extinguishant. U.S. Pat. No. 2,375,832, granted May 15, 1945, to E. Tyden, discloses an automatic fire protection sprinkler head in which a lever is held in position by a retainer which is in turn adhesively held to the spreader portion of the head by a fusible material. Under fire conditions, the fusible material melts to allow the lever to displace the retainer and release the cap on the nozzle. The lateral stress in the lever under normal conditions is described as being resisted by the normal friction between the spreader and the retainer and the adhesive quality of the fusible material. U.S. Pat. No. 3,669,302, granted June 13, 1972, to M. Markarian, discloses an electrolytic capacitor that has a vent opening sealed by a fusible material.